Fuel cell powerplants produce electric power by electrochemically consuming a fuel and an oxidant in one or more electrochemical cells. The oxidant may be pure oxygen or a mixture of gases containing oxygen, such as air. The fuel may be hydrogen.
Each fuel cell generally has electrodes for reacting the gases, such as an anode electrode for fuel and a cathode electrode for an oxidant. The cathode electrode is spaced from the anode electrode. A matrix saturated with electrolyte is disposed between the electrodes.
Each electrode includes a substrate. The substrate has a catalyst layer disposed on the side of the substrate which faces the electrolyte matrix. In some instances an electrolyte reservoir layer, such as an electrolyte reservoir plate, is on the other side of the substrate and is capable of providing electrolyte through small pores in the reservoir plate to the substrate. These electrolyte reservoir plates may have channels or passageways behind the substrate for carrying reactant gases, such as channels for carrying gaseous fuel to the anode and channels for carrying gaseous oxidant to the cathode. For example, these channels might extend between parallel ribs on the substrate side of the electrolyte reservoir plate.
A separator plate on the other side of the electrolyte reservoir plate provides a barrier to the transfer of electrolyte and prevents mixing of the fuel and oxidant gases in adjacent cells.
Another acceptable construction is to have the electrode substrate act both as an electrolyte reservoir plate and as an electrode substrate with channels on the separator side of the substrate.
U.S. Pat. No. 4,564,427 issued to Gruver entitled "Circulating Electrolyte Electrochemical Cell Having Gas Depolarized Cathode with Hydrophobic Barrier Layer" has a cathode which includes a porous plate for a substrate. A barrier layer of a fluorocarbon polymer containing carbon particles is bonded to the porous substrate plate and a catalyst layer is applied to the barrier layer. The barrier layer blocks the flow of electrolyte from passing through the catalyst layer to the substrate while permitting the flow of a reactant gas from the substrate through the barrier layer to the catalyst layer.
Other examples of electrolyte reservoir layers are shown in commonly owned U.S. Pat. Nos. 3,779,811; 3,905,832; 4,035,551; 4,038,463; 4,064,207; 4,080,413; 4,064,322; 4,185,145; and 4,374,906.
Several of these patents show the electrolyte reservoir layer as an electrode substrate. In addition to accommodating changes in acid volume due to electrolyte evaporation and changes in operating conditions of the cell, electrode substrates must satisfy several other functional requirements. For example, the substrate must be a good electrical conductor, a good thermal conductor and have adequate structural strength and corrosion resistance. The substrate must provide support to the catalyst layer and provides a means for the gaseous reactants to pass to the catalyst layer. Finally, the edges of the substrate are often required to function as a wet seal to prevent the escape of reactant gases and electrolyte from the cell.
One way to form a wet seal is to reduce the pore size of the edge region by densifying the edge region, such as through compression during substrate fabrication, and providing a liquid, such as electrolyte to the densified edge region. Densified substrate edge seals are described in commonly owned U.S. Pat. Nos. 4,269,642 and 4,365,008. Experience has shown that the seal density and pore size that can be practically obtained limits the edge seal cross pressure (or, commonly called the bubble pressure) to 3-4 psi.
Another approach to forming the seals is described in U.S. Pat. No. 3,867,206 entitled "Wet Seal for Liquid Electrolyte Fuel Cells" issued to Trocciola et al. which is commonly owned with the present invention. Another example is shown in commonly owned U.S. Pat. No. 4,259,389 issued to Vine entitled "High Pressure-Low Porosity Wet Seal". As discussed in Vine, a seal may be formed in the edge seal region of a porous plate by using a powder filler to provide a denser packing to the region which reduces porosity.
An improved edge seal is described in copending, commonly owned, U.S. Pat. No. 4,652,502 entitled "Porous Plate for an Electrochemical Cell and Method for Making the Porous Plate" filed by Richard D. Breault, a coinventor of this application, and John D. Donahue. In this construction, the electrolyte reservoir layer is a substrate or an electrolyte reservoir plate. The edge seal regions of such porous plates are filled with a high solids, low structure powder which is introduced into the region in suspension form under pressure. The pores of the seal are formed within the edge of the porous plate upon removal of the liquid from the suspension. Such a seal is able to tolerate transient cross-pressure which are an order of magnitude larger than the cross-pressures encountered in the edge region during the normal operation.
Generally, a stack of fuel cells and separator plates are used in performing the electrochemical reaction. As a result of the electrochemical reaction, the fuel cell stack produces electric power, a reactant product, and waste heat. The stack includes a cooling system for removing the waste heat from the fuel cell stack. The cooling system has a coolant and conduits for the coolant disposed in cooler holders to form coolers within the stack. Heat is transferred by the cooler holders from the fuel cells to the conduits and from the conduits to the coolant.
The cooler holder must be electrically and thermally conductive and may be permeable to gas. An example of such a cooler holder is shown in U.S. Pat. No. 4,245,009 issued to Guthrie entitled "Porous Coolant Tube Holder for Fuel Cell Stack". Alternatively, the cooler holder might be impermeable to gas. An example of such a cooler holder is shown in U.S. Pat. No. 3,990,913 issued to Tuschner entitled "Phosphoric Acid Heat Transfer Material". In Tuschner, the cooler holder serves the double function of cooler holder and separator plate.
As discussed, separator plates prevent the mixing of the fuel gas, such as hydrogen, disposed on one side of the separator plate, with an oxidant, such as air, disposed on the other side of the separator plate. Separator plates must be highly impermeable to gases such as hydrogen and oxygen, and thermally and electrically conductive to pass heat and electrical current through the fuel cell stack. In addition, separator plates must also tolerate the severe corrosive atmosphere formed by the electrolyte of the fuel cell, such as hot phosphoric acid, while preventing electrolyte transfer from cell to cell. Finally, separator plates, like cooler holders, must be strong, particularly in terms of flexural strength, which is a measure of the ability of the separator plate to withstand high pressure loads, differential thermal expansion of mating components, and numerous thermal cycles without cracking or breaking.
An example of a method for making separator plates for electrochemical cells is discussed in U.S. Pat. No. 4,360,485 issued to Emanuelson et al., the disclosure of which is hereby incorporated by reference. In this method, the separator plate is formed by molding and then graphitizing a mixture of preferably 50 percent high purity graphite powder and 50 percent carbonizable thermosetting phenolic resin. In particular, Emanuelson discusses forming a well blended mixture of the appropriate resin and graphite powder. The mixture is then distributed in a mold. The mold is compacted under pressure and temperature to melt and partially cure the resin and to form the plate. Typically, such a plate must be heated to for example, to one-thousand degrees (1000.degree. C.) Celsius to convert the phenolic resin to carbon and then graphitized by heating to two-thousand seven hundred (2700.degree. C.) Celsius to provide required corrosion resistance.
The separator plate, because it is a separate component, adds complexity and expense to the manufacture of a fuel cell stack. Efforts have been directed at eliminating such components by providing a seal structure within the porous plates. One example of such a seal structure is shown in U.S. Pat. No. 4,756,981 issued to Breault et al. entitled "Seal Structure for an Electrochemical Cell". Breault discloses a seal region for adjacent porous plates which performs the function of a separator plate with a hydrophobic liquid barrier and a hydrophillic gas barrier.
Other efforts have been directed at eliminating such components by bonding together adjacent plates. For example, a gas separator disposed between the adjacent cathode and anode porous members might be a gas impermeable layer as discussed in U.S. Pat. No. 4,129,685 issued to Damiano entitled "Fuel Cell Structure". In Damiano, two porous members may provide a flow path for the flow of a reactant gas and may be bonded to each other by the gas separator layer that is a thick or thin coating.
U.S. Pat. No. 4,505,992 issued to Dettling et al. entitled an "Integral Gas Seal for Fuel Gas Distribution Assemblies and Method of Fabrication" is another example of such constructions. The gas distribution plate members are bonded together at their interface with a sealant material which extends into the pores of at least one of said porous plates. The sealant material may be selected from the group consisting of fluorinated ethylene-propylene, polysulphone, polyethersulfone, polyphenylsulphone, perflorinated alkoxy tetrafluoroethylene, and mixtures thereof.
Dettling describes a fabrication process for forming the integral assembly of the two porous plates. The process includes providing two porous plates and a layer of sealant material between the plates. The plates and layer of sealant material are subjected to pressure and elevated temperature to melt the layer. As a result, the material in the layer impregnates the porous plates as it melts flowing into the pores to bond the plates together and to seal each plate along the interface against gas transfer.
As noted in Dettling, the pressure applied to the two carbon plates must be great enough to force the facing surfaces of the plates together but not so great as to damage the underlying structure of the plates.
The above art notwithstanding, scientists and engineers are still seeking to develop seal structures for use between the porous plates of electrochemical cells such as the integral separator plates or other plates in abutting contact.